Anharmonicity and the Eigen-Zundel Dilemma in the IR Spectrum of the Protonated 21 Water Cluster

The infrared anharmonic spectra for the H+(H2O)(3), H+(H2O)(4), and H+(H2O)(21) water clusters have been reported using vibrational second-order perturbation theory at the B3LYP level with 6-31+G(d) and 6-311++G(3df,3pd) basis sets. The anharmonicity results crucial for the evaluation of the protonated water clusters and the anharmonic corrections can be larger than 500 cm(-1), resulting in a shift of the H3O+ asymmetric stretchings near the region of 2000 cm(-1).